Free machining alloy



United States Patent Office 3,192,039 FREE MACHINING ALLOY Kermit J. Goda, Jr., Leesport, and John J. Gross, Reading,

Pa., assignors to The Carpenter Steel Company, Reading, Pa., a corporation of New Jersey No Drawing. Filed Aug. 5, 1963, Ser. No. 300,071 Claims. (Cl. 75-128) terial may be machined. Metal machinability is a complex and not fully understood property. However, the manifestations thereof are readily recognized by a skilled artisan from the manner in which the alloy is machined by cutting tools in such operations as turning, milling, broaching, threading, reaming, sawing or grinding. Free machining alloys arecharacterized, among other things,

,by the relatively lower degree of friction or gumminess and hence freer cutting action by the tool; by the small chips removed from the work, and the manner by which the chips fall free from and do not adhere to the tool.

Of the many factors which alfect machinability, the composition of the alloy appears to be the most significant because of its effect upon the structure, processing, and mechanical properties of the alloy. I

Metallurgists have long sought to improve machinability of alloys .by modifyingtheir composition or form. For example, for the purpose of improving machinability,

varying amounts of one or more of such elements as carbon, phosphorus, sulfur, lead, selenium, tellurium,

'arsenic, zirconium and bismuth have beenincluded in alloys.

Sulfur, selenium, tellurium and others of these elements are believed to affect machinability when present in the form of a sulfide, selenide or telluride, respective ly, and for this reason one or more of the elements aluminum, chromium, manganese and molybdenum may be included to form such compounds.

The'free machining additives hitherto used have had relatively limited usefulness for various reasons. Those compositions in which one .or more of the foregoing additives have been utilized with success to provide improved machinability, often had a deleterious effect upon other desired, properties. For example, in the case of chromium stainless steel such as A.I.S.I. types 410, 420,

430 as well as others, the addition of. about 0.3% sulfur results in a marked reduction in the corrosion resistance of the composition although a substantial improvement in free machinability is achieved. r

The present invention stems from our discovery that boron mononitride '(BN) dispersed in the matrix of 30% chromium stainless steel, an alloy which is thachinable with some difiiculty, markedly improves its machinability. This improved free machinability is obtained without the objectionable loss in corrosion resistance normally associated with the addition of such elements as sulfur to chromium stainless steel to'render the same' free machining. On the other hand, when corrosion resistance is of secondary importance to free machinability,

boron mononitride in addition to one or'rnore of the aforementioned free' machining additives may be used to providefigreatl y enhanced free machinability.

The foregoing as well as additional advantages ofthe present'invention are achieved by providing a chromium stainless steel alloy comprising in the approximate amounts indicated (here and throughout this application properties of the composition.

,throughout the matrix. H worked, the boron mononitride appears as black, elon- 3,192,039 Patented June 29, 1965 all concentration percentages are given as percent by weight unless otherwise indicated):

v Percent Carbon vUp to 1.5 Manganese Up to 2 Silicon Up to 3 Chromium 10 to 30 Boron mononitride .05 to 1 In addition, there may also be included varying amounts of other elements, as for example: up to about 3% nickel, up to about 1.5% molybdenum and'up to 1.5% copper, the remainder being substantially iron. Here and elsewhere in the present application, it is intended by the phrase, the remainder substantially iron not to exclude the presence of other elements in amounts ranging from several thousandths of one percent up to about 3% which may be included in keeping with good metallurgical practices or which enhance other desired Normally, phosphorus and sulfur are held to residual amounts. However, when desired as further free machining additives, up to about .5% sulfur and phosphorus as well as selenium may also be included.

In amounts below about 0.05 boron mononitride is not present in sufficient quantity to provide the desired degree of free machinability in our alloy. On the other vhand, the capability of our steel to retain the nitrogen necessary to combine with boron to form boronmononitride necessitates the use of special techniques, as will be more fully pointed out hereinafter, when upwards of about .3% boron mononitride is desired, and above 1% boron mononitride it may be extremely diflicult if not impossible to make sound steel in commercial quantities, even when presently known special techniques are utilized.

The dispersion of boron mononitride may be formed in our composition by adding'the required amounts of boron and. nitrogen uncombined as boron; mononitride employing conventional techniques for melting and casting an ingot having a desired analysis. When the boron and nitrogen are added separately, that is, uncombined as boron mononitride, thev ingot in its as -cast condition contains no more thanv an ineffective amount of boron mononitride, that is,'less than .05%. Apparently the boron-mononitride reaction, if it takes place at all, does so onlyslowly at the elevated melting temperatures and it'is necessary to'hold the cast ingot at temperatures from about 1700 F.'to 2300 F. for from about 1 to 24 hours the longer times being required at the lower temperatures, in order to ensur'formation of the desired amounts of boron mononitride. While the ingot may be heat treated to cause the desired formation of boron mononitride the :heat treatment maybe conveniently incorporated in the heating cycles incidental'to hot working. -When' the boron .mononitride is formed *by heat treating the as-cast ingot, the boron mononitride is .apparent in photomicrographs as generally black, spheroidal" particles f distributed After the metal hasbeen'hot :in the hot worked material but is distributed throughout amount of retained nitrogen may be increased by carrying out the melting and casting of the constituents under a nitrogen atmosphere as brought out, for example, in US. Patent No. 2,865,736. By means of the process set forth in that patent, the usually attained nitrogen content of less than about 3% may be increased to about .5 or more.

In accordance with a further feature of the present invention, the boron mononitride, instead of being formed in situ, may be added in the form of the solid boron mononitride compound. This may be accomplished by adding solid boron mononitride, as, for example, a powder, to the furnace during melting, by seeding the teeming stream or by putting the boron nitride on the bottom of the ingot mold before teeming.

The properties of boron mononitride make it possible to add the solid boron mononitride to the molten metal without breakdown of the compound into its separate constituents. This is advantageous in attaining the dispersion of boron mononitride in alloys whose properties are sensitive to and are impaired by the presence of borides and nitrides which may be formed by the boron and nitrogen remaining when the boron mononitride reaction by which the compound is formed in situ in the heated solid metal is not carried to completion or when strong boride or nitride formers are present in the analysis.

The boron mononitride dispersed in the matrix of our alloy is a soft compound having a layered crystal structure. When an etched or unetched specimen of the alloy is examined under a microscope, the boron mononitride is readily detected because of its black color. On the other hand, boron uncombined with nitrogen, that is combined with other elements as borides, shows up as an extremely light colored inclusion, which characteristically is hard and brittle.

Boron mononitride is highly insoluble in an acid medium such as a hot aqueous solution of sulfuric acid having a 30% by weight concentration of sulfuric acid. Thus, the amount of boron mononitride present may be found quantitatively by determining the amount of boron and nitrogen which is insoluble in the sulfuric acid. The boron mononitride may also be extracted from the alloy by conventional electrochemical processing.

A preferred range which is especially well suited for use in the mass production of corrosion resistant fastening devices, such as screws, comprises in the approximate amounts indicated:

Percent Carbon .07 to .15 Manganese .40 to .80 Silicon .40 to .60 Chromium 11 to 14 Boron mononitride .05 to .25

Example 1 An alloy was melted in the usual manner and cast into an ingot having the following analysis.

Percent Carbon .12 Manganese .45 Silicon .33 Phosphorus .011 Sulfur .029 Chromium 12.20

Percent Nickel .41

Boron mononitride .16

Boron:

Sol .08

Insol. .07

Total .15

Nitrogen:

Sol .05

Insol. .09

Total .1 In connection with the boron mononitride content, the amounts of boron and nitrogen present, but not combined as boron mononitride, are indicated as soluble (Sol.) boron and nitrogen, respectively, while boron and nitrogen combined as boron mononitride are indicated as insoluble (Insol.). This notation relates to the aforementioned method for determinating the amount of boron mononitride present in which sulfuric acid is utilized to solubilize the uncombined boron and nitrogen. The total boron and nitrogen refers to the combined soluble and insoluble amounts of these elements.

In the preparation of Example 1, boron and nitrogen were introduced into the melt by adding a master ferrous alloy containing boron, such as ferro-boron (FeB) and one containing nitrogen, such as nitrided ferro-chromium (N FeCr). Melting and casting into an ingot was carried out in keeping with conventional practices.

The analytic methods utilized were capable of detecting as little as .001% boron mononitride and yet the presence of boron mononitride could not be detected in the as-cast ingots. We found that it is necessary to maintain our alloy at an elevated temperature in order for the boronv and nitrogen to interact to form boron mononitride. It may be well to note here that an important advantage of the present invention resides in the fact that a treating temperature may be selected from the range of about 1700 F. to 2300 F. and a duration which is best suited for the particular analysis being processed. In practice, it has been found that heat treating for a period of up to about 8 hours at a temperature of about 2100 F. to 2300" F. is preferred for providing maximum amounts of boron mononitride.

After being melted and cast, the ingot was hot worked, annealed and then machined to provide specimens suitable for testing. Incidental to hot working, the ingot was heated for from 8 to 10 hours at about 2200" F. to 2250 F.

It should also be noted that, to the extent that the boron mononitride reaction is not carried to completion, there is a residue of boron uncombined as boron mononitride which forms metal borides in our alloy. The formation of an excess of metal borides in any significant amount in our alloy is undesired because of such ill-effects which may result therefrom as embrittlement and poor machinability, as well as impaired ductility, impact strength and cold workability.

The machinability of specimens of the alloy of Example 1 was determined as the average depth of penetration in inches into the specimens attained under carefully controlled conditions. While there is no universally accepted standard for measuring machinability, the free machining values were obtained by measuring the depth of penetration into the specimens by a quarter inch drill, in a time interval of 15 seconds, with the drill rotating at about 670 rpm. under constant torque. Before the start of each drilling operation, the drill mounted in a conventional drill press was brought against the surface of the specimen, where it was maintained by a constant weight of pounds. The depth of penetration was measured with a micrometer and the average depth in inches of seven drilled holes in the specimens of the alloy of Example 1 gave an average penetration of .266 inch.

On the other hand, whenspecimens of A.I.S.I. 410 alloy were similarly prepared and tested, the average penetration was only .158 inch. For comparison purposes, it may be noted that the A.I.S.I. type 410 alloy had the following analysis:

Percent Carbon .114 Manganese .58 Silicon v .39 Phosphorus v .015 Sulfur .019 Chromium 12.47 Nickel .35

with the remainder iron, except for incidental impurities in keeping with good metallurgical practices.

Our alloy, of which Example 1 is illustrative, is hardenable in the usual way and may have an essentially martensitic microstructure or may have a two phase microstructure of both martensite and ferrite.

A further preferred range of our alloy which is not intended to be hardenable and has an essentially ferrite microstructure comprises in the approximate amounts indicated:

Percent Carbon Up to .10 Manganese .40 to .80 Silicon .2 to .6 Chromium 16 to 18 Boron Mononitride .05 to .25

the remainder substantially iron as before. In addition,

up to about .60% molybdenum may be included as well as further free machining additives such as up to about .35% sulfur. Usually nickel, copper and phosphorus are present only as incidental impurities. When corrosion resistance is not a primary concern, the higher carbon levels, up to about 1.5%, may be used and, in addition, up to about .5% sulfur, phosphorus and selenium may be included in our alloy.

Example 2 An alloy of this preferred analysis was prepared, havthe remainder substantially iron, except for incidental impurities.

Specimens of the alloy of Example 2 were prepared and tested as was described in connection with Example 1. The relative machinability is indicated by an average depth of penetration of .278 inch, as compared to an average depth of penetration of .158 inch obtained with specimens of A.I.S.I. type 430 which were similarly prepaved and tested and having the following analysis:

the remainder being substantially iron except for incidental impurities.

It is an advantage of our invention that improved machinability is obtained without objectionably affecting other desired properties of our alloy. In particular, the loss in corrosion resistance normally associated with the use of such free machining additives as sulfur is avoided. Furthermore, when corrosion resistance of the product is not of primary concern, boron mononitride and hitherto utilized free machining additives, such as up to about .5% sulfur, phosphorus and selenium may be used together to provide a unique degree of machinability.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.

We claim:

1, A stainless steel alloy characterized in its heat treated condition by good free machinability consisting essentially of up to about 1.5% carbon, up to about 2% manganese, up to about 3% silicon, from about 10% to 30% chromium, from about .05% to 1% boron mononitride, up to about 3% nickel, up to about 1.5 molybdenum, up to about 1.5 copper, up to about .5% sulfur, up to about .5 phosphorus, up to about .5% selenium, and the remainder consisting essentially of iron, said boron mononitride being dispersed throughout the matrix of the alloy.

2. A stainless steel alloy characterized in its heat treated condition by good free machinability consisting essentially of from about .07% to .15% carbon, from about .4% to .8% manganese, from about .4% to .6% silicon, from about 11% to 14% chromium, up to about .5% nickel, from about .05 to 25% boron mononitride, up to about .5 sulfur, up to about .5 phosphorus, up to about .5 selenium, up to about .6% molybdenum, and the remainder consisting essentially of iron, said boron mononitride being dispersed throughout the matrix of the alloy.

3. A stainless steel alloy characterized in its heat treated condition by good free machinability consisting essentially of from about .03% to 1.5% carbon, from about .4% to .8% manganese, from about .2% to .6% silicon, from about 16% to 18% chromium, from about .05% to .25% boron mononitride, up to about .6% molybdenum, up to about .5% sulfur, up to about .5% phosphorus, up to about .5 selenium, and the remainder consisting essentially of iron, said boron mononitride being dispersed throughout the matrix of the'alloy.

4. A stainless steel alloy characterized in its heat treated condition by good free machinability consisting essentially of about .12% carbon, about .45'% manganese, about 33% silicon, about 12.20% chromium, about .41% nickel, about .15% boron mononitride, and the remainder consisting essentially of iron, except for incidental impurities, said boron mononitride being dispersed throughout the matrix of the alloy.

5. A stainless steel alloy characterized in its heat treated condition by good free machinability consisting essentially of about 036% carbon, about .46% manganese, about '2 8 sulfur, about 17.48% chromium, about .11% nickel, about 2,999,749 9/61 Saunders et al. 75-58 .21% boron rnononi tride, and the rernaindor consisting FOREIGN PATENTS essentrally of H011, szud boron mononltrlde bemg dispersed throughout the matrix of the alloy. 593,342 3/60 Canada- OTHER REFERENCES I 1C't d b th E \eferences N y e (xammer P11101112 Metallurgra Itahana, v01. 51, January 1959,

UNITED STATES PATENTS pages 31-34, published by Stefano Pinelli, via A. Bordoni, 2,283,299 5/42 Lisdale 75 15s x 2, Milan, Italy. 2,388,215 10/45 Murphy 75-123 2,432,619 12/47 Franks et a1 75-428 10 AVID L- RECK, Primary Examiner. 

1. A STAINLESS STEEL ALLOY CHARACTERIZED IN ITS HEAT TREATED CONDITION BY GOOD FREE MACHINABILITY CONSISTING ESSENTIALLY OF UP TO ABOUT 1.5% CARBON, UP TO ABOUT 2% MANGANESE, UP TO ABOUT 3% SILICON, FROM ABOUT 10% TO 30% CHROMIUM, FROM ABOUT .05% TO 1% BORON MONONITRIDE, UP TO ABOUT 3% NICKEL, UP TO ABOUT 1.5% MOLYBDENUM, UP TO ABOUT 1.5% COPPER, UP TO ABOUT .5% SULFUR, UP TO ABOUT .5% PHOSPHORUS, UP TO ABOUT .5% SELENIUM, AND THE REMAINDER CONSISTING ESSENTIALLY OF IRON, SAID BORON MONONITRIDE BEING DISPERSED THROUGHOUT THE MATRIX OF THE ALLOY. 